Method of heat treating iron and steel sheets



Feb. 17,1931. M. cbx ETAL METHOD OF HEAT TREATING IRON AND STEEL SHEETS Filed Jan. 31, 1929 t .w W X m 0 1 3 C m 8 I m M m y e w .H WM UQSVQ Qx wag o D. OH n O 0 I .83 58 mean 0 Patented Feb. .17, 1931 UNITED STATES PATENT OFFICE HERBERT M. COX, OF CANTON, AND CARY M. SMITH, OF MASSILLON, OHIO, ASSIGNORS, BY MESNE ASSIGNMENTS, TO REPUBLIC STEEL CORPORATION, OF YOUNGSTOWN,

OHIO, A CORPORATION OF NEW JERSEY METHOD-OFHEAT TREATING IRON AND STEEL SHEETS Application filed January 31, 1929. Serial No. 336,497.

This invention relates in general to the manfacture of iron and steel sheets, and more particularly to the heat treatment or nor: malizin of such sheets for the purpose of developing certain desirable physical and mechanical properties therein.

As is well known, a sheet which has been normalized under the ordinary practice of normalizing, as for example in an open annealing furnace, has better drawing and stamping qualities than a box-annealed sheet, but owing to its stifl'ness, is likely to wrinkle in forming, and its adaptation for difficult drawing and stamping operations has therefore been somewhat limited.

A box-annealed sheet, on the other hand, has the required degree of softness, but when subjected to diflicult drawing and stamping operations, is likely to break or tear.

In recent years, a demand has arisen, particularly in the automobile industry, for a sheet in which is combined the drawing and stamping qualities of a normalized sheet with the softness obtainable in a box-annealed sheet, and it is a primary object of the present invention to providea method of imparting to low-carbon steel sheets certain qualities as to hardness and structure, which will enable these sheets to be subjected to deep drawing, stamping and other severe forming operations, without danger of wrinkling, breaklng; tearing or cracking. I

In accordance with the invention, low-carbon sheets of steel, as produced by ordinary hot rolling methods, are first subjected to a preheating treatment, the object of which 1s to promote grain growth in the metal and to promote spheroidization of the carbides present in the steel so as to decrease their hardening effect on the final product. This preheatin or annealing treatment is preferably carrie on at a temperature near the lower critical point of the metal, and for a sufficient length of time to produce the aforementioned eifec ts.

The sheets are then heated to a maximum temperature somewhat above the upper critical point of the metal, and are retained at'this temperature for a sufiicient length oftime to cause a rearrangement in the grain structure and solution of the carbides present for the purpose of destroying the strains and banded conditions which have been induced in the sheets as a result of the hot rolling operations.

point of the metal, and are retained at this temperature for a suflicient length of time to induce a maximum degree of. softness in the sheets. Following this treatment, the sheets are allowed to cool slowly and are discharged from the furnace at a temperature not in excess of 1100? F.

Example As an example of the above described heat treatment, attention'is directed to the curve shown in the accompanying drawing, forming a part of this specification, and showing the cycle of treatment as applied to hot rolled 22-gau'ge, open steel sheets, having a carbon content varying from .07% to 12%.

The sheets were conveyed on rolls through a continuous open annealmg furnace, approximately 200 feet long, and divided into a number of zones of different temperatures.

In the first or preheating zone, the sheets were rapidly heated to a temperature of 1325 F., and retained at this temperature for approximately three minutes. The effects of this treatment were a marked grain growth in the metal, and 'spheroidization of the carbides present. 7

The sheets were then heated to a temperature of 1800 F., and retained at this temperbeen induced in the sheets as a result of the hot rolling operations which they had under gone.

Following this treatment, the sheets were rapidly cooled to a temperature of 1100 F., and retained at this temperature for about one minute in order to insure that all parts of the sheet were at the same temperature. The effect of this rapid cooling was the production of a small grain structure of the desired type in the finished sheets.

The sheets were then reheated to a temperature of 1375 F., and retained at this temperature for about four minutes. This final treatment induced a maximum degree of softness in the sheets. The sheets were permitted to cool slowly and were discharged from the furnace at a temperature of about 1000 F.

The sheets, thus treated, were then subjected to a series of physical and mechanical tests, and were found to have a degree of softness and stiffness, which made them admirably adapted for the diflicult deep drawing and stamping operations frequently encountered in the manufacture of automobile bodies and the like.

B low carbon iron and steel sheets, as here in described, it is intended to include iron and steel containing up to about 20% carbon, as well as iron and steel containing up to about 20% carbon and in addition small proportions of alloying ingredients, such as copper, molybdenum, etc.

As the gauges of sheets vary to a considerable extent, the conditions under which they are hot rolled will vary to some extent, and

the strains, banded conditions, grain struc ture, etc. will likewise vary in accordance with the conditions under which the sheets have been rolled. In addition, the upper critical or Ac point of low carbon steels varies from approximately 1650 F. for steel containing 05% carbon to approximately 1550 F. for steel containing 20% carbon. All of these conditions may necessitate a variation in the temperatures employed for the various steps of the heat treatment, as well as a variation in the time required for each step of the treatment. In order to practice these variations it may be necessary to emplo a longer furnace than the one above describe The temperature of the preheating treatment may vary from 1250 F. to 1400 F., and the time of treatment in this temperature range may vary from about one-half to six minutes.

The temperature of maximum heating need be but slightly above the upper critical point of the metal. A temperature of 1800 F. has been found to be suitable under most conditions, but the temperature may vary from 1600 F. to 1800 1*. without departing from the scope of the invention. The time of treat ment at the maximum temperature may likewise vary from two to six minutes.

The rapid cooling of the sheets from the maximum temperature may proceed to any temperature near or below the lower critical point of the metal, but in practice the lowest amams temperature may vary from 1200 F. to 110U F. Owing to the necessity. of reheating the sheets, they need be maintained at this low temperature only momentarily, but in prac-- tice, in order to insure all parts of the sheet being at a uniform temperature, it is desirable to maintain the sheets at this temperature for about a minute. Here again, however, there may be a variation from a small fraction of a minute up to about two minutes.

In reheating the sheets, the temperature to which they are reheated may vary from 1250 F. to 1400" F., and the time'of treatment in this temperature range may vary from two to eight minutes.

Following the reheating treatment, the sheets may be slowly cooled and discharged from the furnace into the atmosphere at a temperature not in excess of 1100 F.

Having described our invention, and in what manner the same is to be practiced, we claim:

1. The method of heat treating iron and steel sheets which comprises heating the sheets to a temperature within the range 1250 F .1400 F., maintaining the sheets at this temperature for a period varying from about one-half to six minutes, then heating the sheets to a temperature within the range 1600 F .1800 F., maintaining the sheets at said temperature for a period varying from two to six minutes, then cooling the sheets rapidly to a temperature within the range 1200 F.1l00 F., maintaining the sheets at this temperature for a period varying from a small fraction of a minute to two minutes,

then reheating the sheets to a temperature within the range 1250. F.1400 F., maintaining the sheets at this temperature for a period varying from two to eight minutes, and then permitting the sheets to cool to atmospheric temperature.

2. The method of-heat treating iron and steel sheets which comprises heatin the sheets to a temperature of about 1325 F maintaining the sheets at this temperature for about three minutes, then heating the sheets to a temperature of about 1800 F., maintaining the sheets at this temperature for about three minutes, then cooling the sheets rapidly to a temperature of about 1100 F., maintaining the sheets at this temperature for about one minute, then reheating the sheets to a temperature of about 137 5 F., maintaining the sheets at this temperature for about four minutes, and then permitting the sheets to coolto atmospheric temperature.

3. The method of-heat treating iron and steel sheets which comprises heating the sheets to a temperature near the lower critical point of the metal, maintaining the sheets at this temperature for a period varying from about one-half to six minutes, then heating the sheets to a temperature above the upper critlcal point of the metal, maintaining the sheets at this temperature for a period varying from two to six minutes, then cooling the sheets rapidly to a temperature below the lower critical point of the metal, maintaining the sheets at this temperature for a period varying from a small fraction of a minute to two minutes, then reheating the sheets to a temperature near the lower critical point of the metal, maintaining the sheets at this temperature for a period varying from 2 to 8 minutes and then permitting the sheets to cool to atmospheric temperature.

In testimony whereof we a-flix our signatures.

HERBERT M. COX,

CARY M. SMITH. 

